1. Field of the Invention
The present invention relates to a semiconductor device including a photoelectric conversion portion and a transistor for signal processing and to a method for manufacturing the semiconductor device.
2. Description of the Related Art
FIG. 6 shows an example of a conventional photoelectric conversion semiconductor device. In FIG. 6, reference numeral 101 denotes a photoelectric conversion portion, and 102 denotes a control portion that controls a signal obtained by photoelectric conversion.
Reference numeral 107 denotes a first conduction type semiconductor substrate. The semiconductor substrate 107 is made of silicon whose impurity concentration is adjusted to about 1xc3x971020 atms/cm3 by including boron as P-type impurities. Reference numeral 108 denotes a first conduction type intrinsic semiconductor layer (hereinafter, also referred to as I layer). The intrinsic semiconductor layer 108 is formed on the semiconductor substrate 107 with silicon that includes boron as P-type impurities in a concentration of about 1xc3x971012 atms/cm3 to 1xc3x971013 atms/cm3. Reference numeral 109 denotes a second conduction type layer. The second conduction type layer 109 is formed on the intrinsic semiconductor layer 108 with silicon whose impurity concentration is adjusted by including phosphorus as N-type impurities. This continuous structure of P-type layerxe2x80x94I layerxe2x80x94N-type layer constitutes a PIN diode for photoelectric conversion. A second conduction type diffusion layer 115 is used as an anode, and a first conduction type diffusion layer 116 is used as a cathode.
The control portion 102 includes a NPN bipolar transistor 103, a PNP bipolar transistor 104, a P-channel MIS transistor 105, and a N-channel MIS transistor 106.
Reference numeral 110 denotes a diffusion isolation region, which separates the photoelectric conversion portion 101 and the control portion 102 by a PN junction, and further separates the NPN bipolar transistor 103 and the PNP bipolar transistor 104.
In the NPN bipolar transistor 103, a collector 123 is formed in the second conduction type layer 109, a base 122 is formed by using boron as impurities, and an emitter 121 is formed by using arsenic as impurities.
In the PNP bipolar transistor 104, a collector 126 is formed by using boron as impurities, a base 125 is formed by using phosphorus as impurities, and an emitter 124 is formed by using boron as impurities.
In the P-channel MIS transistor 105, a source/drain 128 is formed by using boron as P-type impurities. A gate insulating film 112 is formed with a silicon oxide film. A gate electrode 127 is formed on the gate insulating film 112 with polycrystalline silicon that includes phosphorus as N-type impurities.
In the N-channel MIS transistor 106, a P-type impurity region 111 is formed by using boron as P-type impurities. A source/drain 130 is formed in the P-type impurity region 111 by using arsenic as N-type impurities. A gate insulating film 112 is formed with a silicon oxide film. A gate electrode 129 is formed on the gate insulating film 112 with polycrystalline silicon that includes phosphorus as N-type impurities.
Reference numeral 120 denotes an insulator isolation portion, which separates the N-channel MIS transistor 106 and the P-channel MIS transistor 105 by a silicon oxide film.
In this photoelectric conversion semiconductor device, current generated by light entering the photoelectric conversion portion 101 is taken out of the cathode electrode 116, and then converted into a signal by a circuit that is formed as a combination of the NPN bipolar transistor 103, the PNP bipolar transistor 104, the N-channel MIS transistor 106, the P-channel MIS transistor 105, and the like.
In a data reading apparatus for an optical disk such as a compact disk, the market demand for high-speed reading of the optical disk has grown recently. A photoelectric conversion device that converts an optical signal into an electric signal is used in a read portion of the data reading apparatus. Therefore, the achievement of high-frequency property of the photoelectric conversion device is indispensable for meeting the market demand. In the conventional photoelectric conversion device in FIG. 6, P-type impurities contained in the semiconductor substrate 107 diffuse to the side of the intrinsic semiconductor layer 108 during the manufacturing process, and a portion in which the impurity profile changes gradually is formed at the contact portion between the semiconductor substrate 107 and the intrinsic semiconductor layer 108. Therefore, in addition to the current that is generated due to carriers in a depletion layer when light enters, a current component is produced due to a diffusion of carriers generated in the portion of impurity profile gradient into the depletion layer after a delay. Consequently, time resolution is reduced.
To achieve a photoelectric conversion device with a good high frequency property, a measure for improving the response characteristics of the PIN diode has been employed, e.g., by adjusting the concentration of the first impurity in the semiconductor substrate 107, the thickness of the intrinsic semiconductor layer 108, and the thickness of the second impurity layer 109, or a measure for reducing a wiring resistance component has been employed.
Though these measures are effective in improving the high frequency property of the PIN diode, they have an adverse effect on the characteristics of the bipolar transistors and MIS transistors in the control portion 102. Examples of such an adverse effect include a degradation of the element isolation property, such as leakage current and a decrease of withstand voltage, at the PN junctions between the collector of the bipolar transistor and the semiconductor substrate 107 and between the source and drain of the MIS transistor and the semiconductor substrate 107, an increase in parasitic capacitance, and the formation of a parasitic transistor. This leads to a decrease in the level of a converted electric signal by the photoelectric conversion portion, which in turn causes degradation of performance, such as processing accuracy and processing speed, for the signal processing portion and a reduction in yield.
It is an object of the present invention to provide a semiconductor device that can form bipolar transistors and MIS transistors for signal processing and adjust the characteristics of the transistors easily without being affected by the conditions of formation of a PIN diode for photoelectric conversion, such as the impurity concentration of a semiconductor substrate and the thickness of an intrinsic semiconductor layer, and a method for manufacturing the semiconductor device.
A semiconductor device of the present invention includes the following: a semiconductor substrate of a first conduction type; an intrinsic semiconductor layer of the first conduction type formed on the semiconductor substrate, the intrinsic semiconductor layer having a lower impurity concentration than that of the semiconductor substrate; a first semiconductor layer of a second conduction type formed on the intrinsic semiconductor layer; a first impurity layer of the first conduction type formed in the first semiconductor layer of the second conduction type; and a bipolar transistor and a MIS transistor formed in the first semiconductor layer of the second conduction type. The laminated structure of the semiconductor substrate, the intrinsic semiconductor layer, and the first semiconductor layer provides a diode for photoelectric conversion. A first insulator layer is formed in at least a portion below the bipolar transistor and a second insulator layer is formed in at least a portion below the MIS transistor.
According to this configuration, the insulator layers are formed respectively below the bipolar transistor and the MIS transistor, so that the transistors can be isolated electrically from the semiconductor substrate. This leads to improvements in the performance of the transistors and in the characteristics of the PIN diode. Therefore, a high-performance PIN diode can be achieved to make it easier to provide a photoelectric conversion device with high-frequency property, which increases the yield of the photoelectric conversion device.
A method for manufacturing a semiconductor device of the present invention includes the following: forming an intrinsic semiconductor layer of a first conduction type on a semiconductor substrate of the first conduction type, the intrinsic semiconductor layer having a lower impurity concentration than that of the semiconductor substrate; forming a first semiconductor layer of a second conduction type on the intrinsic semiconductor layer; forming a first impurity layer of the first conduction type in the first semiconductor layer of the second conduction type; forming a bipolar transistor in a portion of the first semiconductor layer of the second conduction type, the bipolar transistor including a collector diffusion layer, a base diffusion layer, and an emitter diffusion layer; and forming a MIS transistor in a portion of the first semiconductor layer of the second conduction type, the MIS transistor including a source diffusion layer and a drain diffusion layer. The laminated structure of the semiconductor substrate, the intrinsic semiconductor layer, and the first semiconductor layer provides a diode for photoelectric conversion. A first insulator layer is formed in at least a portion below the bipolar transistor and a second insulator layer is formed in at least a portion below the MIS transistor.